This invention relates to plug removal apparatus and more particularly to apparatus for removing plugs from heat exchanger tubes.
In tube type heat exchangers, a first fluid flows through the tubes of the heat exchanger while a second fluid surrounds the outside of the tubes such that heat exchange occurs between the two fluids. Occasionally, one of the tubes can become defective such that a leak occurs therein which allows the two fluids to mingle. When this occurs, it is sometimes necessary to plug the tube so that the fluid does not flow through the tube thereby preventing leakage from the tube.
In nuclear reactor power plants, the tube type heat exchangers are commonly referred to as steam generators. When a defect occurs in the tubes of the nuclear steam generator that allows the coolant in the tubes to mingle with the coolant outside of the tubes, a more significant problem arises. Not only does this situation create an ineffective heat exchanger, but it also creates a radioactive contamination problem. Since the fluid flowing in the tubes of a nuclear steam generator is generally radioactive, it is important that it not be allowed to leak from the tubes and contaminate the fluid surrounding the tubes. Therefore, when a leak occurs in a nuclear steam generator heat exchange tube, the heat exchange tube is plugged so that the coolant is not permitted to flow through the tube. This prevents contamination of the fluid surrounding the tubes.
There are several kinds of plugs that may be used to plug the heat exchange tubes. One such device used to plug heat exchange tubes in nuclear reactors is an explosive plugging device. With the explosive plugging devices, a metal plug is inserted in the heat exchange tube with an explosive contained within the plug. When the explosive is detonated, the plug is forced into close contact with the inside of the tube thus blocking flow through the tube. One problem associated with explosive plugging is that should it become necessary to replace the defective tube or a defective plug, the explosive plug must be drilled out which is a time consuming procedure.
Another device used to plug heat exchange tubes is generally referred to as a mechanical plug. The mechanical plug comprises a cylindrical member closed at one end and having a tapered inner bore. An expander member is disposed in the cylindrical member such that when the expander member is drawn along the longitudinal axis of the cylindrical member, the cylindrical member is forced into close contact with the heat exchange tube due to the relative tapers of the inner surface of the cylindrical member and the expander member. In this type of device, no explosives are necessary to create the close contact between the plug and the heat exchange tube. Rather, the expansion of the plug is caused by the relative movement of the expander member in the plug. This gives the mechanical plug the characteristic of being able to be removed without being drilled out. In order to remove the mechanical plug, it is, thus, first desirable to relieve the internal pressure on the plug by moving the expander member to a position in the plug in which the tapered diameters of the plug and expander member are not in contact. Next, with the pressure on the plug relieved, the plug may then be pulled from the heat exchange tube.
In order to quickly remove the mechanical plug from the heat exchange tube, it would be desirable to have a mechanism that is capable of both relieving the pressure on the plug and pulling the plug from the heat exchange tube. Therefore, what is needed is apparatus that is capable of quickly relieving the pressure on the plug so as to loosen the plug in the heat exchange tube and that is capable of extracting the plug from the heat exchange tube.